The invention relates to devices and methods for dispensing a drug from a Metered Dose Inhaler (MDI) canister into a stream of air supplied through the inspiratory path between a ventilator and an endotracheal tube in the bronchial passage of a patient.
Drugs dispensed from metered dose inhalers usually consist of very finely divided particles, typically in the one to eight micron range. The medication particles are suspended in pressurized liquid Freon propellent which is under pressure in the MDI canister. Upon actuation, a metered dose of the drug and Freon propellent is ejected through the outlet tube of the canister and a medication nozzle of a MDI ventilation dispenser 1A as shown in FIG. 3. As the mixture of drug and Freon propellent is ejected from an exit port 8 of the medication nozzle 5 into the body 2 of MDI ventilation dispenser 1A, it is accelerated to such high velocity that shear forces with the nearly stationary ambient air cause the propellent to break up into many small, rapidly evaporating droplets, each of which contains hundreds or thousands of drug particles. The exit port 8 of a ventilation dispenser 1A (prior art FIG. 3) typically has a diameter of approximately 0.6 millimeters. The rapid evaporation of Freon droplets produces rapid cooling, which may result in condensation of water that replaces the evaporating Freon, resulting in some medication particles being contained in water droplets. In any case, the rapidly evaporating droplets travel at high speed and expand into a plume 10 along a longitudinal axis of MDI ventilation dispenser 1A. The size and maximum diameter of the plume is determined by when the droplets become completely evaporated.
Therefore, the rate at which the droplets of propellent can evaporate (thereby releasing dry particles of the drug which can be carried by the inspiratory air stream flowing through the inspiratory tubing, a wye connector, and the endotracheal tube into the lungs of the patient) is limited by how fast heat can be absorbed by the droplets from the surrounding air and the wall of the inspiratory path. (Note that evaporation is a phase transition that requires absorption of heat which must come from the droplets, which cool very quickly and therefore must absorb heat from the surrounding air for evaporation to continue.)
As is well known, a major problem with MDI ventilator dispensers is that the expanding plume 10 of large un-evaporated droplets and the medication particles contained therein impinge against the inside walls of the connecting tubing that constitutes the inspiratory path. Most or all of the drug particles contained in each impinging droplet adhere to the wall of the inspiratory tubing and therefore are not carried by the inspiratory air stream into the lungs of the patient. To prevent droplets of plume 10 from impinging on the inner walls of the inspiratory path, devices called "spacers" sometimes are used. Such spacers have enlarged diameters into which the plume can continue to expand until all of the medication-containing droplets have evaporated, as described in U.S. Pat. Nos. 4,470,412 (Nowaki), 4,484,577 (Sackner), 4,790,305 (Zoltan), and 5,012,803 (Foley). Unfortunately, use of such spawcers increases the volume of the inspiratory circuit and therefore also increases the "compressibility" of the ventilator circuit.
This is undesirable because compression of air in the inspiratory circuit, as the ventilator pushes a "breath" of air into the lungs of the patient, results in a reduction in the amount of air actually delivered to the lungs of the patient. Such compression also can result in a reduction in the amount of medication delivered via the inspiratory air stream into the lungs of the patient. Thus, such compression can produce uncertainty both in the amount of oxygen and the amount of medication received by the lungs of the patient, especially if the patient happens to be a small child.
Furthermore, use of spacers also increases the amount of surface area in the inspiratory circuit on which condensation can occur, increasing the amount of condensate that accumulates and which must be drained. The expense of using spacers and the discomfort to the patient of additional weight in the apparatus connected to the end of the tracheal tube also are factors that favor avoiding the use of spacers.
Thus, there is an unmet need for an improved device and technique to reduce or eliminate the loss of medication particles contained in Freon or water droplets which impinge upon the inner surfaces of the inspiratory path.